In blow molding hollow plastic articles, it is frequently necessary to produce hollow articles that require a multi-part mold so that the parts of the mold can be separated from each other in a manner that will permit the release of the finished hollow article.
Hollow articles may have many shapes. For example, they may have the form of a cylinder with a relatively flat bottom, and in this case the finished article can be released after the blow molding stage by separating the two halves of the mold in a direction perpendicular to the axis of the article. Since many articles produced by blow molding, however, have a neck or rim that is narrower than the body of the article, there may be undercuts at the neck or rim so that simple axial removal of the article is out of the question.
Very often the article is not cylindrical, but is oval or has a rim in cross-section, so that opening the mold to remove the article requires a surface area that corresponds to the particular shape of the article. Furthermore, there may be cases in which the article does not have a comparatively flat or even bottom, but in which, for example, a shape has been selected for the bottom that resembles that of a champagne bottle. Such a bottle has a bottom in which there is a part that projects into the interior space to a considerable extent. In such a case, simple opening of the mold or separating the two halves of the mold is not adequate for the removal of the article. Accordingly, a third part is needed for the mold in order to form the bottom. This part is either moved away from the article in an axial direction, while the two other parts of the mold that serve to form the sides are moved perpendicularly to the axis, or the two side parts of the mold are separated, whereupon the article remains on the part of the mold forming the bottom, and is separated from it in an axial direction.
In order to carry out the movements mentioned above, the parts of the mold must be held in a mechanism which serves to unite the parts into the mold (the mold clamp), and to lock them together so that sufficiently large forces are available to be able to counteract the pressure generated in the mold cavity during the forming process (that is, blowing under pressure) and to be able to separate the parts of the mold from each other. Such a mechanism is called a mold clamp, and such a mold clamp can be improved in accordance with the findings of the present invention.
Thus far, mold clamps have consisted of a pair of platens to which the halves of the mold were fastened. For the purpose of back-and-forth movement under the action of fluid activated cylinders, or with the aid of a toggle- or cam-mechanism, these platens may be positioned perpendicular to the mold face. The same mechanism was used not only for movement, but also for locking the parts of the mold.
If a third mold component was used for the reasons mentioned above, it was actuated by a fluid cylinder of its own, or mechanically linked to the side parts of the mold. There are many devices on the market for performing the movements mentioned above.
The blowing of hollow articles is usually carried out at a pressure of 10 bar or less, and this involves pressures at which the mechanisms described above worked satisfactorily. Thus far, too, the overall cycle of such blow molding operations was regulated in such a way that the speed at which the blow mold moved and the article was removed did not matter. Recently, however, articles have been made of plastics with a considerable resistance to deformation at the prevailing conditions used in blowing, and this resistance even tends to increase in the course of the blowing operation. Higher pressures were used, therefore, up to 40 bar, while the operating speed of the clamp parts was increased and the length of time needed for the removal of the article was reduced, because improvements in the blow molding process reduced the overall cycle significantly.
The clamping devices described above are not adequate for the tasks imposed upon them during the process so that not quite all the advantages can be obtained from the more recent blow molding methods, nor are they capable of keeping the mold in the closed position against the high blowing pressures that are required.